ARM: mach-at91: fix timer.o compile condition

The AT91 architecture now has two possible timer blocks, the old PIT timer
and the new PIT64B.
The timer.c file has an old non DM driver that works for platforms
that do not use the ATMEL_PIT_TIMER DM-based driver.
Update the Makefile to select this old driver in case neither of the
ATMEL_PIT_TIMER and the MCHP_PIT64B_TIMER are selected.

Suggested-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>